I have an old Sigma 300mm f4 that I bought back in the film days. It wasn't fully compatible with the digital bodies so I recently had it chipped. It works like a charm again. The lens is reasonably sharp at f5.6. It has a distance window and a lens hood. Overall - it's not a bad lens. I'm considering getting one of the old teleconverters (the EX x2.0) which in theory should be compatible with the lens I have. I will loose autofocus and exposure information according to Sigma. However, I guess this applies to a non-chipped version of the lens. Is there a risk of damage if I try the TC? It will effectively be a f8 lens so I guess I won't care that much if I cannot change the aperture for instance. An attractive, and 10-times as expensive, solution would be to get the Canon EF 300mm f4 IS version and a teleconverter to go. I would get the compatibility and the IS... Is it sharper?